Hearing aid



Jan. 11, 1938. H. A. PEARSON ET AL HEARING AID 2 Sheets-Sheet l Filed Oct. 4, 1954 INVNTORS I H14. Pearson E C. Nz'c/mll 'des ATTORNEY Jan. 11, 1938. H. A. PEARSON ET AL HEARING AID 2 Sheets-Sheet 2 FiI Led Oct. 4, 1934 s s a R Y 4 mn w M Nw R ww m m C T E 0 5 9 H Patented Jan. 11, 1938 ilNl'iED sra'rss ATENT orries HEARING AID Application October 4, 1934, Serial No. 746,794

for concealed and inconspicuous wear and able to restore to them the ability to hear and thus act like normal persons. Accordingly, soon after the invention of the telephone, efforts were started to make a hearing-aid utilizing the telephone principle.

Small portable hearing-aids that could be worn concealed by a person of impaired hearing have been known for many years. They consisted of a transmitting microphone connected in series with a small battery to a telephone receiver worn on the ear of the person with impaired hearing for producing in the ear channel sound vibrations of greater strength than those that would ordinarily reach it. Such instruments are only of limited benefit because a microphone actuated by sound waves can only produce current oscillations of small magnitude and the sound vibrations developed by the receiver can be strengthened only to a very limited degree. high excitation threshold of a microphone transmitter makes the intensity of the current changes produced by it fall off very steeply with the distance so that hearing is possible only in the immediate vicinity of the speakerl The unsatisfactory action of such devices brought about many efforts to provide the deafened with an amplifier connected between the microphone transmitter and the receiver in order to enable sufficient amplificaiton of the speech currents to secure an enlarged output from the telephone receiver. Since the excessive bulk of the batteries and associated apparatus made it impossible to use electron amplifying tubes in connection with portable hearing-aid devices suitable for concealed wear by a deafened person essential for a practical hearing-aid, these efforts have been concentrated on the development of a small microphone amplifier for securing a substantial amplification of the currents supplied to the telephone receiver by the transmitter microphone of an ordinary hearing-aid.

Very serious obstacles have for many years prevented the development of a satisfactory practical microphone amplifier. Microphone ainplifiers depend for their action on the variation of the contact resistance between a large number of carbon particles. These contact variations are very erratic and produce many disturbing effects. Difiiculties also are encountered at the contact surfaces of the electrodes with the carbon particles. Much trouble was also caused by over-modulation which occurs when vibrations are imparted to some carbon particles with an 4 Claims.

This invention relates to hearing-aid microphone amplifiers and it has among its objects improved portable microphone-amplifier hearingaid devices which provide increased sound am- .5 plification and sound intensity beyond the limits to which heretofore microphone amplifiers used in connection with hearing-aids had to be confined in order to avoid noises accompanying the action of the carbon particles of amplifier miif) crophones.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawings wherein Fig. l is a diagrammatic view illustrating a hearing-aid embodying the invention;

Fig. 2 is an enlarged horizontal sectional view of the amplifier unit of the hearing-aid of Fig. :30, 1 along line 2-4 of Fig. 3;

Fig. 3 is an elevational view of the interior of the amplifier unit with the cover removed;

Fig. 4 is a perspective View of the condenser element which is a part of the amplifier unit shown in Figs. 2 and 3;

Figs. 5 and 6 are enlarged vertical sectional views of a bone oscillator and a telephone re ceiver provided with a condenser element in accordance with the invention;

Figs. 7 and 8 are views similar to Figs. 1 and 2 showing another embodiment of the invention;

Fig. 9 is a sectional view of the amplifier unit along line 9-9 of Fig. 8; and

Fig. 10 is a perspective view of the condenser element of the amplifier unit of Figs. 7 to 9.

Microphone amplifiers in which weak speech frequency currents of an input circuit are supplied to an actuating winding of an electromagnetic vibrator used in telephone receivers to impart corresponding vibrations to a carbon microphone and generate corresponding amplified currents in an output circuit connected to the microphone have been a subject of much investigation and development work in the telephone field for many years prior to the invention of electron tubes, but this work did not succeed and was abandoned with the introduction of the electron tube amplifiers.

It has been known for many years past that many people who during the first part of their lives heard normally and at a later time, after having acquired the characteristics of normal persons, have through sickness or similar causes lost their hearing to a lesser or greater degree were in need of a hearlng-aid device suitable The intensity at which they do not oscillate around a fixed position of equilibrium in continuous contact with each other, but move freely relatively to each other and break their contact while carrying current causing sparking between the contact surfaces.

Many of these difiiculties have been overcome in microphone amplifiers for hearing-aid devices disclosed in Reissue Patent No. 19,246 of July 17, 1934 \vluch gives a substantial amount of good amplification of sound frequency current without distortion and without disturbing noises during a long useful life.

One of the essential features of the microphone disclosed in said patent resides in the relatively stiff diaphragm, preferably of steel, for imparting the vibratory movement to the movable carbon diaphragm electrode acting on the carbon gr nules within the microphone chamber. By -ilriiig the diaphragm suificiently stiff, amplitude of the vibrations may be maintained within limits at which no over-modulation occurs within the microphone chamber and the disturbing noises due to the breaking of the contacts between particles while carrying current are avoided. While such amplifiers enable satisfactory amplification of speech and music without distortion and without disturbing noises, the magnitude of their amplification is limited by t requirement that the amplitude of the vi ation of the movable diaphragm carbon electrode shall be limited to a relatively small value.

The magnitude of the amplifying effect depends on the amplitude of the movable contact siu'iace acting on the carbons in the microphone chamber and the force imparted to the carbon particles. The disturbances caused by the break between the carbon balls due to an increase or" the amplitude of the movable contact diaphragm and the increased force imparted to the particles made any improvement in the amplifying action worthless because the contact breaks and sparking between the carbon particles produced current disturbances which obliterated the speech current oscillations and made the reproduced speech unintelligible.

The present invention overcomes the limitations restricting the amplitude of the microphone diaphragm Vibations and permits the construction of carbon microphone amplifiers in which the vibratory diaphragm microphone electrode operates with amplitudes which produce overmodulated vibrations of the carbon particles that would ordinarily produce disturbing noises. In accordance with the invention, these disturbances are eliminated by connecting the amplifier microphone electrodes to a condenser or" substantial capacity proportioned to suppress the effect of currents producing the disturbing noises due to the action or" the carbon particles subjected to over-modulated vibrations.

This suppression of the disturbing currents produced by the increase of the amplitude and the forces with wlruch the movable diaphragm electrode act on the carbon particles in the microphone chamber is preferably effected by connecting asmall electrolytic condenser to the electrodes of the amplifier chamber. In the preferred arrangement the condenser element is made a part of one of the units of the hearingaids as used heretofore. The capacity of the condenser may vary within wide limits. A condenser of 0.2 microfarad is of substantial benefit in reducing the disturbing noises. Condensers of capacities of the order of to l microfarad or even more are preferable when properly combined with the other elements of the hearingaid so as to maintain a substantial amplification effect and at the same time suppress the disturbing noises.

An essential feature of the present invention is the utilization of the large amplification obtained by the improved microphone amplifier disclosed herein for increasing the amount of power which can be applied to a small wearable hearlug-inducing sound receiver for producing by bone conduction satisfactory hearing in the hearing center of a person. It has been known for many years past that many persons born with satisfactory hearing who during the first part of their lives heard normally and at a later time, after having acquired the characteristics of normal persons, have through sickness or similar causes lost their hearing to a lesser or greater degree, and become subject to maladjustment as a result of this inferiority, could be restored to their normal character by providing them with a hearing-aid device suitable for concealed and inconspicuous wear and able to make them hear well, and free them from the strain to which they are imposed by their impairment.

It has also been known that hearing-aids which impart to a deafened person hearing by means of an air-conduction telephone receiver are only of a. limited benefit to most of the deafened people because the intensity of the sound produced in the ear channel is limited by the inherent delicacy of the normal hearing elements in the ear of a person. If an attempt is made to produce in the ear channel of a person sound vibrations of high intensity, the mechanical elements of the hearing organs are likely to become injured and increase the deafness rather than aid the hearing. Thus, a person of normal hearing to whom high intensity sound vibrations are imparted with a telephone receiver will not be able to hear in the way he hears normal sound, but the excessive sound vibrations will cause him a great deal of pain and noise preventing him from understanding the transmitted sound.

In imparting hearing by bone-conduction receivers, a greatly increased amount of power may be transmitted to the bone structure of the person without impairing his hearing ability. This makes it possible to operate bone-conduction receivers with sound-frequency currents of much greater intensity.

The microphone amplifier of the present invention enables the operation of bone-conduction receivers with amplified currents of a high intensity which could not be obtained with microphone amplifiers heretofore available. It also enables utilization of portable bone-conduction hearing-aids which develop sound vibrations of an intensity exceeding that permissible with airconduction hearing-aids.

In Fig. 1 is shown a hearing-aid device exemplifying the invention comprising a hearing imparting receiver l0, having an actuating winding l l, a microphone transmitter unit l2 with a control unit 13, a battery l4, and an amplifier unit IS with two supply terminal plug pins l6, ll engaging the positive and negative terminal sleeves I8 and 19 of the battery, the several elements being interconnected by a flexible cord 20 having a transmitter branch with a three-terminal plug 2| engaging the terminals of the transmitter unit I2, an amplifier branch with a three-terminal plug 22 for interchangeably engaging either the terminals of the amplifier unit I5 or the battery sleeves of the battery 14, and a receiver branch 24 having two strand ends provided with plug pins 25 engaging terminal sleeves 21 of the receiver 10 leading to the actuating winding ll thereof.

The receiver 10 may be either a bone-conduction receiver of the type shown in Patent No. 2,062,372, or an air-conduction telephone receiver of the type shown in Patent No. 1,973,410.

The transmitter unit 12 has two microphone electrodes 3|, 32, and its control unit 13 has a resistor 33, a stationary contact member 34, a contact rod 35, and a sliding contact member 36 movable between a position where it makes a conducting connection between the resistor 33, the stationary contact member 34 and the contact rod 35, cutting out the resistor when moved one way, and interrupting the connection between the several elements when moved in the opposite way. One microphone electrode 32 of the transmitter is connected to the stationary electrode 34 of the control unit, and the other microphone electrode 3|, the contact rod 35 and one end of the resistor 33 are connected to three transmitter terminal contacts 31, 38, 39 with which three plug terminals 4|, 42, 43 of the transmitter plug 2| of the cord are in detachable contact engagement.

The amplifier unit I5 comprises a small casing fitting the space above the battery 14 and enclosing a microphonic amplifier, such as disclosed in Reissue Patent No. 19,246 granted July 17, 1934. It comprises an electromagnetic vibrator having a magnetic core structure formed of a central soft iron core member 52 and two permanently mag netized core arms 53 held together by means of a metal strip 54 having its ends attached by means of screws 55 to a rigid pole plate 56, having at its center an annular pole face surrounding the pole face at the end of the central core member 52 and spaced therefrom by an annular gap traversed by the flux induced by the two permanent core members.

The central core member 52 is surrounded by an actuating winding in the form of a coil having one terminal connected through the core body to the clamping strip and another terminal in the form of a contact spring 6| mounted on one side of the coil. A fiat armature of soft magnetic iron bridging the annular core gap is held spaced by a small gap adjacent the pole faces by means of a diaphragm strip 66 of resilient material, such as steel, which has its ends clamped between clamping plates 61 by screws 68 threaded into the pole plate. On the side opposite to the armature the diaphragm strip 66 has attached thereto a carbon diaphragm electrode 10 which lies in front of an opening 1| of a carbon chamber 12 formed in an insulating block 13 secured to pole plate by screws 14. The carbon chamber 12 is filled with small carbon balls and is enclosed on the opposite side by a carbon contact plate 15 which is held in place by means of a strip 16 having one bent-over end attached to one edge of the insulating block 13 and the other end fastened by a screw 18 to the opposite edge of the block 13.

The amplifier unit is attached to inward projections of the housing walls by means of screws 19. The two supply pins l6 and I1 of the amplifier unit are secured to the bottom wall of the casing and arranged for detachable engage ment with the battery terminal sleeves I8 and 19 to provide a good electrical as well as mechanical supporting connection between the amplifier unit and the battery. The amplifier unit has inaddition three terminal sleeves 8|, 82, 83 secured within the end wall of the casing 50 and arranged to engage three terminal pins 84, 85, 86 of the cord plug 22. The inner ends of the three-terminal sleeves 8|, 82, 83 are connected by suitable contact strips 62, 63, 64 provided in the casing to the contact spring terminal at one end of the amplifier winding 60, the supply terminal pin 16 and to the stationary microphone carbon electrode 15, respectively, the other end of the winding 58 and the amplifier microphonediaphragm 10 being connected through the metallic magnetic core and the contact strip 62 to the other supply pin 11 of the amplifier.

The features of construction of the microphone amplifier are more fully described in the Reissue Patent No. 19,246, referred to above.

By interconnecting the transmitter, the receiver, the amplifier and the battery by the cord 20 in the way shown in Fig. 1, sound waves impinging upon the transmitter microphone 3| will produce oscillatory sound frequency currents through a primary circuit extending from the positive battery pole, through contact sleeve 18, terminal l5, plug pin 85, then through the cord lead extending therefrom to the transmitter terminal contact 38, through contact rod 35, contact rod 34, the transmitter microphone electrodes 32, 3|, transmitter contact terminal 31,

by way of the cord lead to plug pin 86, and there- 1 from by way of winding 63 and supply terminal 11 to the negative pole of the battery 14. The oscillatory currents through the actuating winding 60 impart to the armature 65 and to the carbon diaphragm 10 attached thereto vibratory motion and produce corresponding variations of the resistance formed by the carbon balls of the amplifier microphone chamber 12, generating corresponding amplified current oscillations in the secondary circuit of the hearing-aid extending from the positive terminal of the battery, through terminal pins 16, over the common circuit portion through cord pin 85, cord lead extending therefrom to transmitter contact 38, contact rod 35, contact 36, by way of resistor 33 to transmitter contact 39, plug pin 42, by way of the cord lead extending therefrom including the receiver winding ll, through the amplifier plug pin 84 and the amplifier microphone electrodes 15, 10 and amplifier supply terminal l1 to the 7? negative pole of the battery.

In microphone amplifiers of the type described above as made heretofore, the amplitude of the movable contact member acting on the carbon particles in the microphone chambers must be limited to a value where the carbons do not cause excessive noise due to the contact interruption and resulting sparking produced by large amplitudes of the diaphragm in addition to the inherently present carbon hiss.

These limitations are overcome in accordance with the present invention by connecting to the amplifier microphone electrodes an electrolytic condenser 93 of a capacity sufficient to suppress the disturbing high frequency currents which have heretofore prevented fuller use of the amplification range obtainable with microphone amplifiers and at the same time does not appreciably divert the oscillatory currents important for the production of the sound vibrations from the receiver Winding.

The small electrolytic condenser unit 90 oocupies very little space within the amplifier casing, and is shown mounted adjacent the amplifier mechanism within the interior of the amplifier casing 50. As shown in Fig. 4, the condenser unit has a capacity of microfarad, its fiat sides are by /8 inch, and its thickness is about A inch. It is made of an inner metallic electrode 9i of a material, such as aluminum, surrounded by a second similar electrode 92 of opposite polarity with an intervening layer of an electrolytic substance 93, and is of the type described in Patents Nos. 1,891,207, 1,912,223,

- 1,815,768 and 1,963,049 which are generally available in trade. The condenser has its negative terminal lead 05 clamped under the strip 54 of the amplifier unit, and its positive terminal lead is clamped by screws '58 to contact strip 16, thereby connecting the condenser to the two electrode plates 10 and of the microphone chamber.

in practical experiments with microphone amplifi it has been found that most of the disturbing noises are produced by large oscillation amplitudes of the diaphragm electrode in the frequency range between about 300 and. 800- cycles. By connecting a condenser as described above which has a high impedance for oscillations within the range up to about 1,000 cycles er second and a low impedance for frequencies above 3,000 cycles, greatly increased amplification without disturbing noises is obtained by increasing the oscillation amplitudes of the carbon diaphragm electrode 70 to values at which such increased amplitudes would have produced in prior art microphone amplifiers disturbances destroying or materially reducing the intelligibility of the transmitted speech. lhe amplitudes of the oscillations of the diaphragm electrode 10 is increased by proper design of the vibrator unit which imparts the oscillations to the electrode '50, for instance, by using in the vibrator unit described above a diaphragm strip 66 having a stiffness which is only above onehalf of the stiffness which the diaphragm strip would require to prevent disturbing noises in the absence of a condenser unit connected in accordance with the invention.

Favorable conditions are obtained by making the condenser impedance equal to the impedance of the receiver windin; connected in the output circuit of the amplifier at about 3,000 cycles per second. For instance, for a bonecon-duction receiver winding having at a current frequency of 3,000 cycles an impedance of ohms, a condenser of a capacity of microfarad will have at about 3,000 cycles the same impedance as the receiver. At about 1,000 cycles, which is close to the region of frequencies important for the reproduction of speech, such receiver will have an impedance of about 45 ohms and the same condenser will have an impedance of about 320 ohms. Accordingly, the use of such condenser will give no noticeable frequency discrimination in normal speech reproduction while it will secure a substantial elemination of the noise. In using under the same conditions a condenser of 1 microfarad, the condenser impedance will be equal to the receiver impedance of 75 ohms at 2,000 cycles. While such a condenser would slightly reduce '18 high frequency components of speech, the speech reproduction is still excellent and the noise suppression very complete.

With receivers of the same construction, a condenser of 2 microfarads gives a substantial improvement without interfering with the use.

Contrary to expectations, experiments with microphone amplifiers have shown that a high resistance connected in series with the microphone greatly increases the disturbing high frequency currents when contacts between carbon particles are broken and sparking is produced by too large amplitudes of the movable diaphragm electrode acting on the particles within the chamber. By decreasing the resistance in the circuit of the carbon chamber to low values, the wave form of the current in the output circuit is considerably improved and the disturbing noises suppressed. Since the condenser acts as a low impedance at high frequencies where the oscillator impedance is very high, its action is of great benefit in cutting down the noise producing effect of the microphone contacts.

The efiect of the condenser is important not only in suppressing the currents produced by sparking between carbon particles on breaking the contacts, but is also of beneficial effect in preventing sparking or cutting down substantially its duration so that it cannot produce a harmful effect.

In connecting electrolytic condensers to the electrodes of the microphone chamber, great care should be taken to apply the terminals of the condenser with the proper polarity in the circuit extending from the battery. This is particularly important in connection with electrolytic condensers which have a greater insulating eifect in one direction than in the other.

Instead of placing the electrolytic condenser within the microphone casing and directly connecting its terminals to the electrodes of the microphone chamber, the condenser may be provided as a separate unit detachable by means of plugs to the circuit leading to the amplifier microphone electrodes, or may be housed in one of the casings of the other elements of the portable hearing-aid. Thus, for instance, an electrolytic condenser of suitable size lill may be mounted within the casing l02 of a bone-conduction receiver I03 shown in Fig. 5 and described in Patent No. 2,062,372. The electrolytic condenser I01 has its two terminals I05 and I05 connected to the terminal sleeves 10'! through which connection is made to the actuating coil I08 of the receiver. The condenser, which has an insulating coating of paraffin, for instance, on its exterior surface may be held clamped in position between the cover of the receiver and an extension from the base thereof as shown at H38.

In a similar way the condenser may be moiuited in a housing of a telephone receiver as shown in Fig. 6 where a small electrolytic condenser I I0 is placed in a cavity 1 l l within the bottom of the telephone casing of a telephone receiver of the type disclosed in Patent No. 1,973,410, the terminal leads H2, H3 of the condenser being connected to the terminal sleeves H4 of the receiver to which the actuating coil H5 is connected.

The disturbing noise-creating currents are caused not only by the erratic action of the carbon microphone i0, 15 of the amplifier unit in the secondary circuit of the amplifier, but also by the similar action of the carbon contacts of the transmitter microphone 3!, 32 constituting with the actuating coil 60 a primary circuit for imparting the oscillatory action to the diaphragm electrode 10 of the amplifier microphone. In the embodiment of the invention shown in Figs. 7 to 9, the disturbing effect of the carbon contacts of the transmitter microphone as well as the amplifier microphone are suppressed by connecting to the microphones of the two circuits condensers of substantial capacity, proportioned to suppress the disturbing effects without impairing the effecting of the generation of amplified sound producing currents.

As shown in the drawings, the two condensers may be made in the form of a single unit I20 having two condenser cells formed between a common aluminum electrode l2i connected to the common lead of the two microphones to the supply terminal connected to the negative terminal of the battery I23 and two distinct coopcrating electrodes I25, 126 of opposite polarity connected through leads 96' and I2! to the chamber electrode 15 of the amplifier microphone and to the contact spring terminal SI of the amplifier coil 60, respectively. The con denser element formed of electrode E25 and the associated portion of electrode |2l is made and proportioned to constitute a capacity which will have an impedance of the order of the impedance of the receiver H] at about 3,000 cycles, and a substantially greater impedance than the impedance of the oscillator In at frequencies of about 1,000 cycles per second. In a similar way the condenser element formed of electrode I26 and the associated portion of electrode i2! is made and proportioned to constitute a capacity which will have an impedance of the order of the impedance of the amplifier winding 60 at about 3,000 cycles, and a substantially greater impedance than the impedance of the amplifier winding at frequencies of about 1,000 cycles per second.

An electrolytic condenser in which each condenser unit has a capacity of about to 1 microfarad made in a way similar to the condenser shown in Fig. 4 will be very effective in suppressing the disturbing noises in an amplifier arrangement of the type described above. Such condenser is small in size, and can be readily mounted within the casing I30 enclosing the amplifier elements without in any Way affecting the portability, lightness and inconspicuousness of the hearing-aid.

The features of the invention described above are susceptible of many modifications that will suggest themselve to those skilled in the art.

We claim:

1. In a microphone amplifier actuated by sound frequency input currents for producing corresponding amplified sound frequency currents supplied to an output circuit of a hearing-aid device or the like, an amplifier microphone having a vibratory electrode and carbon granules actuated by the vibratory motion of said electrode connected in said output circuit, a vibratory structure actuated by said input currents for imparting to said microphone electrode a vibratory motion, said amplifier microphone and said vibratory structure being proportioned and constructed to operate said vibratory electrode with an amplitude range producing through over-modulated vibrations of said carbon granules audible current impulses superimposed on said sound current oscillations, and a condenser unit connected parallel to said amplifier microphone having an impedance of the order of the output impedance at a frequency between 1,800 and 4,000 cycles and an impedance two or more times greater than the output impedance at a frequency between 600 and 1,200 cycles for suppressing said superimposed current impulses without materially impairing the intensity of the sound frequency oscillations supplied to the output circuit incident to said over-modulated vibration.

2. In a microphone amplifier actuated by sound frequency input currents for producing corresponding amplified sound frequency currents supplied to an output circuit of a hearing-aid device or the like, an amplifier microphone having a vibratory electrode and carbon granules actuated by the vibratory motion of said electrode connected in said output circuit, a vibratory structure actuated by said input currents for imparting to said microphone electrode a vibratory motion, said amplifier microphone and said vibratory structure being proportioned and constructed to operate said vibratory electrode with an amplitude range producing through overmodulated vibrations of said carbon granules audible current impulses superimposed on said sound oscillations, a condenser unit connected parallel to said amplifier microphone having an impedance of the order of the output impedance at a frequency of the order of 2,500 cycles and an impedance two or more times greater than the output impedance at a frequency of the order of 1,000 cycles for suppressing said superimposed current impulses without materially impairing the intensity of the sound frequency oscillations supplied to the output circuit incident to said over-modulated vibrations, and a housing enclosing said amplifier microphone, said vibratory structure and said condenser unit having cord leads for operatively interconnecting said amplifier michrophone and said vibratory structure with said input and said output circuits.

3. In a microphone amplifier actuated by sound frequency input currents derived from a battery for producing corresponding amplified sound frequency currents supplied to an output circuit of a hearing-aid device or the like, an amplifier michrophone having a vibratory electrode and carbon granules actuated by the vibratory motion of said electrode connected in said output circuit, a vibratory structure actuated by said input currents for imparting to said microphone electrode a vibratory motion, said amplifier microphone and said vibratory structure being proportioned and constructed to operate said vibratory electrode with an amplitude range producing through over-modulated vibrations of said carbon granules audible current impulses superimposed on said sound oscillations, a condenser unit connected parallel to said amplifier microphone having an impedance proportioned and correlated with respect to the impedance of said output circuit to divert said superimposed current impulses from the output circuit without materially impairing the intensity of the sound frequency oscillations supplied to the output circuit incident to said over-modulated vibrations, an inconspicuously small housing enclosing said amplifier michophcne, said vibratory structure and said condenser unit, said housing having detachable terminal connections for mechanically supporting said housing on said battery, and cord leads for operatively interconnecting said amplifier microphone, said vibratory structure and said battery with said input and said output circuits.

. In a microphone amplifier actuated by sound frequency input currents derived from a battery for producing corresponding amplified sound frequency currents supplied to an output circuit of a hearing-aid device or the like, an amplifier microphone having a vibratory electrode carbon granules actuated by vibratory motion of said electrode connected in said output circuit, a vibratory structure actuated by said input currents for imparting to said microphone electrode a vibratory motion, said amplifier microphone and said vibratory structure being proportioned and constructed to operate said contact member with an amplitude range producing through overmodulated vibrations of said carbon granules audible current impulses superimposed on said sound oscillations, an electrolytic condenser unit connected parallel to said amplifier microphone having an impedance of the order of the output impedance at a frequency between 1,800 and 4,000 cycles and an impedance two or more times greater than the output impedance at a frequency between 600 and 1,200 cycles for suppressing said superimposed current impulses Without ma-.

terially impairing the intensity of the sound frequency oscillations supplied to the output circuit incident to said over-modulated vibrations, and an inconspicuously small housing enclosing said amplifier microphone, said vibratory structure and said condenser unit, said housing having detachable terminal connections for operatively interconnecting said battery, said amplifier microphone and said vibratory structure with said input and output circuits.

HARRY A. PEARSON. EMMANUEL CHRIST NICHOLIDES. 

